#include <iostream>
#include <vector>
#include <queue>
#include <stack>
#include <algorithm>
using namespace std;

struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

class Solution {
public:
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        vector<vector<int>> res;
        queue<TreeNode *> que;
        if(root) que.push(root);
        while(!que.empty()){
            size_t size = que.size();
            vector<int> vec;
            while(size--){
                TreeNode * node = que.front();
                que.pop();
                vec.push_back(node->val);

                if (node->left) que.push(node->left);
                if (node->right) que.push(node->right);
            }
            res.push_back(vec);
        }
        reverse(res.begin(), res.end());
        return res;
    }
};